Field of the Invention
The present invention relates to electric motors used for operating pumps, and particularly to those used for operating pumps intended for food or medical use.
The hygiene aspects of pumps for liquids for food use or for medicines in liquid form must be carefully controlled in that the pumped liquid must not come into contact with contaminants. To satisfy this requirement, in such pumps the impeller shaft is not mechanically connected to the motor shaft, but rather the shafts are connected together by electromagnetic coupling.
In particular, the free end of the impeller shaft emerging from the pump carries a pole wheel consisting of radial field electric magnets, the number of poles of which depend on the type of application.
The pole wheel is positioned inside a jacket in the shape of a cup of circular cross-section, and extends from the pump casing to form a chamber isolated from the outside. The jacket has an inner diameter of a few tenths of a millimeter greater than the outer diameter of the pole wheel, and is generally constructed of amagnetic stainless steel.
Outside the jacket there is an outer second pole wheel having a number of poles equal to the number of poles of the inner first pole wheel.
The second pole wheel is mechanically connected to an electric motor shaft in such a manner that upon rotating the motor shaft and hence the outer pole wheel, the inner pole wheel connected to the pump impeller is rotated by induction.
Although the described solution perfectly performs the function for which it is provided, it presents certain drawbacks.
A first drawback is derived from the overall size due to the in-line arrangement of the pump, the electromagnetic coupling and the electric drive motor.
A second drawback is derived from the fact that if the electric motor has a high static torque and good acceleration the inner pole wheel disengages from the outer pole wheel whereby the impeller does not move.
To solve the first drawback pumps have been proposed, one of which is fully described in U.S. Pat. No. 5,197,865, in which the outer pole wheel is replaced by the stator of a stepping motor having a number of stator poles different from the number of rotor poles.
The use of a stepping motor makes it necessary to use Hall probes or similar devices to exactly identify the position of the rotor relative to the stator field to prevent such current absorption as to cause excessive stator heating. In addition, the use of a stepping motor requires the use of a costly and complicated control system which makes the product economically less desirable. There is also the drawback of more complicated equipment construction due essentially to the power conductors for the Hall probes, for which two are required for each phase winding, to be added to the two power conductors for each phase.
Accordingly, the object of the present invention is to overcome the stated drawbacks within the framework of a rational, reliable and low-cost solution.
The present invention attains such object by providing a motor-pump unit in which the motor rotor is mechanically connected to the pump impeller while at the same time being physically isolated from the stator by a jacket made of amagnetic material.
Specifically, the stator, positioned outside the jacket of amagnetic material has a three-phase winding with a number of poles equal to the number of poles of the rotor winding.
To obtain a high static torque the present invention is powered by an electronic power circuit which generates a system of three-phase currents, the frequency of which gradually increases starting from zero, by following an appropriate acceleration ramp.
The electronic power circuit is controlled by a control circuit comprising a microprocessor which measures the counter-electromotive force arising in the stator, starting from a minimum value corresponding to about one quarter of the predetermined normal working r.p.m. In particular, the microprocessor is able to sense when the counter-electromotive force developing in the stator passes through zero, and hence identify the instantaneous position of the rotor. In this manner the motor current absorption can be controlled on the basis of the applied resistant torque. This enables current absorption to be limited by feeding the motor with only that current necessary for operating the load, thereby preventing overheating damage of the electrical circuits.
In a simplified embodiment, the motor of the present invention can be powered by a usual inverter. In this case however, the current absorbed by the motor is always equal to the maximum current required to provide the maximum torque.
The slope of the powering current diagram (ramp) for the motor evidently depends both on the resistant torque and on the moment of inertia of the rotating masses, such as the rotor and the impeller.